1. Field of the Invention
The present invention relates to a polyimide alignment film based on 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and m- or p-phenylene diamines or 1,5-diaminonaphthalenes, which are substituted ortho to the amino groups with alkyl groups containing from 1 to 4 carbon atoms, such as 2,3,5,6-tetramethyl-p-phenylene diamine (DAD) or 2,4,6-trimethyl-m-phenylene diamine (DAM), and to a liquid crystal display device using such an alignment film.
2. Description of the Prior Art
Liquid crystal display (LCD) devices have become increasingly important in displays which require very low consumption of electrical power or where the environment dictates a lightweight, planar, flat viewing surface. Thus, LCD's are used in display devices such as wristwatches, pocket calculators and personal computers, aircraft cockpit displays, etc.
In its simplest form a liquid crystal display device consists of a liquid crystal layer with opposite sides, a set of electrodes on either side of the liquid crystal layer and an alignment layer between each set of electrodes and the liquid crystal layer. The electrodes bearing the alignment layer are supported by substrates typically of glass or plastic. Alignment of the liquid crystal molecules occurs at a certain angle, referred to as the surface tilt angle or simply as the tilt angle, with respect to the plane of the inside of two substrates, e.g. glass plates, plastic sheets, quartz plates or others, which support the electrodes. The inside of the substrates have coatings of sets of transparent electrodes (electrical conductors), usually made of indium-tin oxide (ITO). The sets of electrodes are patterned, e.g. by etching, compatible with the information to be displayed by the LCD and with its driving method. Displays using the TN or the STN effect use electrodes on opposite sides of the liquid crystal layer in order to achieve the predominantly vertical electrical field required for the switching of the liquid crystals in these display modes. The TN effect is e.g. widely exploited in so called active matrix TN displays, which feature electronic active switching elements (e.g. TFTs or diodes) in each pixel. TN-displays are already widely used, for example in monitors for lap-top computers. Another display mode is the in-plane-switching (IPS) mode. Here the electrodes of one pixel are on the same side of the liquid crystal layer and switching is achieved by an essentially horizontal electrical field, i.e. an electrical field which is essentially parallel to the liquid crystal layer. IPS displays are frequently addressed by a matrix of active elements (typically of TFTs). The process of establishing an alignment layer is most easily carried out by applying the orientation material (an organic polymer) via solution casting (spin coating, roller coating, dipping, spraying, printing and/or doctor blading) onto the substrates. After removal of the solvents and/or curing of the polymer layers, in most conventional displays the substrates are usually rubbed or buffed in one direction with cloths to establish an unique optical direction. After rubbing both substrates, they are rotated from 0 to 360 degrees with respect to each other; adhered together using organic adhesives and often appropriate spacers to preserve a constant thickness to a space or gap between the substrates; and filled with various mixtures of liquid crystal materials. At this stage, polarizing films and/or compensation films are often attached to the outside surfaces of the substrates by a lamination process. Finally, electrical connections are made to both substrates in a manner consistent with the electrical and display designs.
Presently the use of rubbed polymer films, i.e. alignment direction and tilt angle controlling films, dominates the process technology used in the production of all categories of liquid crystal displays, and polyimides are the most common alignment films in use today.
However it is a process hardly compatible with the required clean room conditions in the production of most LCDs, especially active matrix addressed LCDs. Utmost care has to be taken to prevent the rubbing wheels or rubbing cylinders, which usually are covered by cloths like velvet, from dusting and giving off tiny threads which both lead to defects in the orientation of the liquid crystals and thus to defective displays. A further problem of the rubbing method is the generation of electrostatic charges which are able to damage the electronic switching elements of the active matrix.
Another method used in the orientation of liquid crystals is the oblique evaporation of inorganic materials like silicon-oxide (SiO.sub.x) onto the substrate surfaces. This method, however, is hardly suited for the mass production of LCDs, especially of LCDs with larger display areas, as handling of the substrates under vacuum is difficult in particular for large substrates. And further, uniform tilt angles are hard to achieve over wide substrate areas. Last but not least, oblique evaporation generally yields rather high surface tilt angles of as much as 30.degree., which are not suited for most practical applications.
A review of conventional alignment controlling techniques is given, for example, by I. Sage in Thermotropic Liquid Crystals, edited by G. W. Gray, John Wiley & Sons, 1987, pages 75 to 77 and by J. M. Geary et al. in Journal of Applied Physics, Vol. 62(10), 1987, pages 4100-4108.
In contrast to these methods, photoalignment of liquid crystal offers significant advantages. Photoalignment of liquid crystals is characterized by the alignment of liquid crystals on alignment surfaces treated by or subjected to light. The expression light here includes not only the visible spectrum of electromagnetic radiation but also the adjacent wavelength regions, especially the UV-range. As a contact-free and especially dust-free method, it is of high interest for the production of liquid crystal displays.
Photoalignment of LCs in LCDs has been proposed by M. Schadt et al., Japanese Journal of Applied Physics 31, p. 2155 (1992) using photo-cross linking of polyvinylcinnamate. However neither the capability of this material for the orientation of liquid crystals, nor especially the stability of the alignment after temperature load was sufficient for practical applications.
Moreover, the tilt angle and its magnitude are very important in the various electro-optic modes and the electro-optic properties of the resulting LCD devices. The stability, legibility and reliability of the LCD are all related to the magnitude and stability of the tilt angle. The tilt angle has to be stable towards high temperature and illumination, and the magnitude of the tilt angle has to be stable to long storage in order to provide a long operational life time for the displays. This holds particularly for the value of the tilt angle obtained after the heat treatment of the display after or during sealing of the cells filled with liquid crystals.
Polyimide films used to control the alignment direction and the tilt angle of the liquid crystal molecules in liquid crystal displays are very thin, generally being on the order of from 100 to 2000 angstroms. In the case of photo aligned orientation layers the alignment is induced in a unique direction of the polyimide polymer by illumination with polarized UV-light. The tilt angle is induced by irradiation of the substrates at a pre-determined angle of incidence. The actual tilt angle obtained is a function of polymer ordering on the surface, the resulting surface energy and the illumination angle and illumination intensity. In addition to these variables, each of the hundreds of commercial liquid crystal formulations interacts differently with a given surface. In general, however, the single most important factor determining the value range of the tilt angle is the intrinsic character of the polyimide used to control this angle. Twisted nematic (TN) LCDs, including active matrix (AM) TN LCDs, such as those used in pocket TV sets and watches, generally require lower tilt angles in the range of 1 to 5 degrees. Supertwisted nematic (STN) LCDs require higher tilt angles, typically between 4 to 30 and particularly between 5 to 15 degrees. Whereas IPS displays, as described e.g. in DE 4000451, EP 0 588 568 and EP 0 644 452, require rather low tilt angles of 0 to 5 degrees and preferably of 0 to 2 degrees.
Thus, polyimide alignment films for liquid crystal displays must exhibit certain key properties including stable and predictable alignment of liquid crystal molecules and a sufficient tilt angle for the envisaged application. In addition, for active matrix displays, the polyimide alignment film must also have a high value of the so-called voltage holding ratio (VHR). The active matrix electrode layer comprises nonlinear addressing elements such as, for example, thin film transistors (TFT), metal-insulator-metal (MIM) diodes or metal-silicon nitride-indium tin oxide (MSI) diodes which are integrated with the image point. In TN displays each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the voltage holding ratio (VHR) which is defined as the ratio of the drop in voltage across an image point in the non-addressed state to the voltage applied. A process for determining the VHR is given, for example, by B. Rieger et al., in Conference Proceedings der Freiburger Arbeitstagung Flussigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg, 1989. The influence of the resistivity of the liquid crystals on the performance of IPS displays is less pronounced than in TN displays however IPS displays cannot tolerate too low VHR values as electro-optical systems having a low or relatively low VHR show insufficient contrast.
For active matrix applications, polyimide alignment films based on 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) as the tetracarboxylic dianhydride component and e.g. 2,3,5,6-tetramethyl-p-phenylene diamine (DAD) as the diamine component are used employing the rubbing method (EP appln. No. 96115528).
Alignment layers for IPS displays are disclosed e.g. in EP 0 644 452, JP 07-043 716, JP 07-261 180 and JP 07 261-181, all of which employ the rubbing method.
Typically, alignment layers for active matrix displays are pre-imidized, soluble polyimides which eliminate the need for high cure temperatures and only require removal of the solvent. The rigidity of the tetracarboxylic dianhydrides disclosed in the reference suggests that none of the resulting polyimides would be expected to have the required solubility in solvents typically used for this process.
U.S. Pat. No. 4,912,197, issued on Mar. 27, 1990, discloses highly soluble, opaque to clear aromatic polyimides derived from 2,2-bis-(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA), which can be partially replaced with 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), and 2,4,6-trimethyl-m-phenylene diamine (DAM) or 2,3,5,6-tetramethyl-p-phenylene diamine (DAD).
U.S. Pat. No. 5,532,110, issued on Jul. 2, 1996, also discloses a photoimageable polyimide coating derived from DAM or DAD and 6FDA which can be replaced with BTDA to a certain extent. The polyimide is used as a coating for microelectronic applications which is imagewise exposed and selctively etched to form a pattern
U.S. Pat. No. 4,629,777, issued on Dec. 16, 1986, describes radiation-sensitive polyimides prepared from ortho-substituted diamines and BTDA, specifically from DAM or DAD and BTDA.
European Patent application 0 365 855, published on May 2, 1990, discloses a polyimide derived from DAD and a tetracarboxylic acid dianhydride such as BTDA. The rubbed polyimide is used as an alignment layer for LCD devices.
However, there is no disclosure in the above documents of a photo alignment layer for a liquid crystal display consisting of a BTDA/DAM or DAD polyimide.